The increasing power of electronic devices is motivating the semiconductor industry to seek effective thermal management solutions. A number of techniques for cooling electronic devices and packages have become widespread. Currently, fin array heat sinks with fans are the most common cooling technique. These devices are typically attached to a package lid or directly to a die. A primary purpose of heat sinks is to increase the area for heat rejection to air.
Another common technique that has emerged in the past decade is the use of heat pipes. The heat pipe has become a widely used thermal management tool in the notebook industry. Most current notebooks utilize heat pipes in their thermal management solution. Their primary purpose is to remove heat from a heat source to a heat sink where the heat is dissipated. Heat pipes are vacuum-tight vessels that are evacuated and partially filled with a small amount of water or other working fluid with a wicking structure. FIG. 1 shows an illustration of how a heat pipe works. As heat is directed into a heat pipe 110 from a heat source 120, fluid evaporates creating a pressure gradient in the heat pipe 110. This forces vapor 130 to flow along the heat pipe 110 to a cooler section or condenser 160 where it condenses. The condensed fluid wicks 140 back to the evaporator 150 near the heat source 120.
Heat removal capacity of the heat pipe is controlled by wicking media and heat pipe geometry, among which important parameters are heat pipe cross-sectional dimensions and heat pipe length. Currently, available heat pipes with 6 cm outer diameter (OD) can only dissipate about 30 to 50 W of heat. High performance electronic devices often dissipate more than 100 W of heat. Therefore, multiple heat pipes are used for thermal management. These heat pipes, usually 3 or more, are often embedded in a cooper enclosure or block to form a cooling module.
The current designs have an important limitation. Due to their OD sizes, the heat pipes are much comparable with a typical die size. Spreading thermal resistance from the die to each individual heat pipe becomes significant. FIG. 2 shows an illustration of multiple heat pipes embedded in a copper block 200 and coupled to a heat spreader 210. The heat spreader 210 sits on top of a heat source 220, such as a chip. A center heat pipe 230 is positioned closest to the heat source 220, with outer heat pipes 240 located farther away from the heat source 220. The center heat pipe 230 and the outer heat pipes 240 have identical boiling points of 50 degrees Celsius. The spreading thermal resistance causes the outer heat pipes 240 to remove much less heat than the center heat pipe 230. The center heat pipe 230 could reach its boiling limit and exceed its heat removal capacity, causing it to “burn out” before the outer heat pipes 240 have approached their full heat removal potential. Therefore, heat load carried by the outer heat pipes 240 will be far less that the amount carried by heat pipes closer to the heat source, such as the center heat pipe 230. Thus, the center heat pipe 230 will approach and exceed its heat load capacity before the outer heat pipes 240 reach their heat load capacity, resulting in a system poorly adapted for handling high heat load.
What is needed is an apparatus for and method of optimizing boiling points of heat pipes to achieve simultaneous onset of boiling.